1. Field of the Invention
This invention relates to apparatus for the analysis of fluidborne particles. For examples, in the study of aerosols, aerosol dispersion and airborne particulate pollution control, there is a requirement for the rapid determination of particle size distribution especially in the diameter range 1 to 10 microns, together with some knowledge of the geometry and symmetry of individual particles. The latter information could, for example, enable particles with spherical symmetry to be identified and thus allow the counting/monitoring of liquid droplets in an environment including other solid, non-spherical particles. In the context of the present specification, the term particles is intended to apply both to solid bodies and to drops of liquid.
It is desirable for such apparatus to be able to count individual particles in a sample at rates of, typically 20,000 particles per second, to be able to distinguish between spherical and non-spherical particles in the sample and to count each type. Another desirable feature is to categorise spherical particles having diameters of a few microns into a number of size bands and also in this connection to classify particle coincidences as `non-spherical` and hence to ignore them in the compilation of size spectra based on the assumption the particle is spherical.
2. Field of Prior Art
The normal techniques for the examination of particles, as used in several instruments available commercially, employ the detection and analysis of electromagnetic radiation scattered by the particles. All such instruments use a mechanical mechanism to drive the sample air through a "sensing volume" where the carried particles are illuminated by the incident electromagnetic radiation. The radiation scattered by the particles is received by one or more detectors which convert the energy to electrical signals from which information may be extracted by appropriate electrical circuits.
Particle analysers are known, for example, as described in UK Patent Application numbers 8619050, 2044951A and U.S. Pat. No. 3,946,239. These all describe analysers which comprise a concave reflector in a scatter chamber, and a flow of sample fluid intercepted by a beam of radiation. The light scattered from individual particles in the fluid is directed by the reflector to radiation collectors and subsequently analysed. All of these, however, suffer from being cumbersome and fragile and consequently not readily portable. Moreover, light scattered at low angles from the particles in the sample is not detected by any of the above prior art systems.
Another analyser is described in UK Patent number 2041516 which discloses a particle analyser of the type described above but which has an additional feature in that the concave reflector has a transparent window in it which is used to collect back scattered light by use of lenses and photomultipliers. The intensity of the back scattered light achieved by this apparatus is very low and passes through collimation means before being collected by photomultipliers. This means that not all back scattered radiation will be detected and the apparatus required to collect the light is complex, cumbersome and relatively expensive. Moreover, the apparatus is not portable.